User operated drug delivery devices are as such known in the prior art. They are typically applicable in circumstances, in which persons without formal medical training, i.e., patients, need to administer an accurate and predefined dose of a medicinal product, such as heparin or insulin. In particular, such devices have application, where a medicinal product is administered on a regular or irregular basis over a short term or long-term period.
In order to accommodate with these demands, such devices have to fulfill a number of requirements. First of all, the device must be robust in construction, yet easy to use in terms of handling and in understanding by the user of its operation and the delivery of the required dose or medicament. The dose setting must be easy and unambiguous. Where the device is to be disposable rather than reducible, the device should be inexpensive to manufacture and easy to dispose. Moreover, the device should be suitable for recycling. To meet these requirements, the number of parts required to assemble the device and the number of material types the device is made from need to be kept to a minimum.
The medicinal product to be dispensed by means of the drug delivery device is typically provided in a disposable or replaceable cartridge, such as a vial, ampoule or carpule comprising a slidably disposed piston to be operably engaged with a piston rod of the drug delivery device's drive mechanism. By applying thrust to the cartridge's piston in distal direction, a predefined dose of the liquid drug can be dispensed and expelled from the cartridge.
Cartridges as they are typically used with drug delivery devices, such as pen-type injectors are typically sealed by means of as sealing septum. Such a septum is commonly designed as rubber stopper providing an air-tight seal but being pierceable by piercing elements such as needles or cannulae.
A typical cartridge holder assembly 10 as known in the prior art is illustrated in cross section in FIG. 1. This cartridge holder assembly 10 of a drug delivery device comprises a cartridge holder 14 adapted to receive a cartridge 12, which is hermetically sealed with a flexible and deformable septum 22. At its lower and distal end section, the cartridge holder 14 is threadedly engaged with a needle mount 16. Said mount or needle holder 16 comprises a threaded cylindrical portion allowing to screw the needle holder 16 on the threaded neck portion of the cartridge holder 14. At its lower and distal section, the mount 16 comprises a flange-like bottom face 17, which in a concentrically inner section holds the injection needle or cannula 20.
During assembly of the needle holder 16, the proximally located tipped end of the needle 20 penetrates the septum 22. In this way, a fluid-transferring connection for the purpose of dose dispensing can be established. Additionally, the distal and free end of the needle 20 can be provided with a replaceable needle cap 19. Also, the entire cartridge holder assembly 10 can be covered and protected by a protective cap 18.
Depending on manufacturing tolerances and the corresponding design of cartridge 12 and cartridge holder 14, an axial gap 24 of variable size is typically formed between the bottom portion 17 of the needle holder 16 and the distal end face of the cartridge 16. Axial size of this free space area 24 may vary, e.g due to manufacturing and assembly tolerances. In particular, during dispensing of a dose of the medicinal fluid contained in the cartridge 12, a respective fluid pressure is built-up, which, due to the elasticity of the septum 22, leads to a respective axial expansion of the septum 22. As a consequence, the septum 22 may almost entirely fill said free space area 24 when a respective fluid pressure inside the cartridge 12 establishes.
Due to the elastic properties of the septum 22, said septum 22 will store elastic energy during dose dispensing. But as soon as the fluid pressure returns to an initial value after a dose dispensing procedure, the septum 22 relaxes to its initial configuration, which is accompanied by a retraction of the expanded section of the septum 22 into the cartridge 12. However, also such a retracting motion may in turn lead to a built-up of a non-negligible fluid pressure enhancement and, as a consequence, a certain amount of medicinal fluid may be supplementally expelled from the cartridge 12, which can be typically observed in the form of droplet formation at the distal tip of the needle 20.